An activated carbon is made from carbon materials such as carbonized coconut shell, petroleum coke or coal coke that is activated to have a porous structure. The activated carbon, which is porous and thus has a large surface area, has been widely used as an absorbent, a catalyst support, and an electrode material for double layer capacitors and lithium secondary batteries. In particular, in order to increase the energy density, i.e., capacitance in an electric double layer capacitor, which may be used in a hybrid car or the like, an activated carbon having fine pores effectively formed thereon, a high crystallinity and a large surface area has been demanded to be used as an electrode material for the capacitor.
For the industrial production of such an activated carbon with effectively formed fine pores that can be used as an electrode material of an electric double layer capacitor, a method for activation has been generally used, in which a carbon material such as petroleum coke and an alkali metal compound such as potassium hydroxide are heated at a temperature of 600 to 1200° C. in an inert gas atmosphere to allow the alkali metal to ingress between and react with the resulting graphite crystal layers (Patent Literature 1). In this type of activation, the alkali metal enters the layered structure wherein condensed polycyclic hydrocarbons are layered, and as the result forms fine pores.
In a method for producing an activated carbon for an electric double layer capacitor electrode by activating carbon material with an alkali activator, the activator is generally used in an amount of 2 to 4 parts by weight of the carbon material to be mixed therewith. In particular, when the intended specific surface area is large ranging from 2000 to 3000 m2/g, the activated carbon is produced necessarily at a large “activator/carbon material” ratio. However, it is desired to minimize the ratio of the alkali activator to be used because the share thereof in the production cost is large.
In mixing of a carbon material and an alkali activator, the carbon material is water-repellant and thus poor in wettability with the alkali activator, which is water-soluble. Simple mixing of these two materials makes the contact therebetween insufficient. A large part of the activator is thus not used for activation reaction, and the resulting activated product (activated carbon) is small in specific surface area.
As a means for bringing these materials into strong contact, methods have been known, wherein the materials are mechanically mixed with a ball mill or a Henschel mixer and wherein the alkali activator is melted and then mixed with the carbon material (Patent Literature 2). However, any of these methods requires the use of an activator in an amount greater than the theoretical amount so as to proceed with the activation reaction efficiently and causes an increase in the production cost.